In some diagnostic and therapeutic techniques, a catheter is inserted into a chamber of the heart and brought into contact with the inner heart wall. For example, in intracardiac radio-frequency (RF) ablation, a catheter having an electrode at its distal tip is inserted through the patient's vascular system into a chamber of the heart. The electrode is brought into contact with a site (or sites) on the endocardium, and electrical activity in the heart chamber is detected by the electrode. Moreover, RF energy may be applied through the catheter to the electrode in order to ablate the heart tissue at the site.
Catheters for mapping and/or ablation typically carry one or more magnetic position sensors for generating signals that are used to determine position coordinates of a distal portion of catheter. For this purpose, magnetic field generators are driven to generate magnetic fields in the vicinity of the patient. Typically, the field generators comprise coils, which are placed below the patient's torso at known positions external to the patient. These coils generate magnetic fields that are sensed by the magnetic position sensor(s) carried in the catheter. The sensor(s) generate electrical signals that are passed to a signal processor via leads that extend through the catheter.
Proper contact between the electrode and the endocardium is necessary in order to achieve the desired diagnostic function and therapeutic effect of the catheter. Excessive pressure, however, may cause undesired damage to the heart tissue and even perforation of the heart wall. For pressure sensing, a catheter typically carries a miniature transmitting coil and three sensing coils on opposing portions of a flexibly-jointed distal tip section. The transmitting coil is aligned with the longitudinal axis of the catheter and three sensing coils are also aligned with the longitudinal axis but positioned at an equal distance from the transmitting coil, and at equally-spaced radial positions about the longitudinal axis of the catheter. The miniature transmitting coil generates a magnetic field sensed by the three sensing coils which generate signals representative of axial displacement and angular deflection between the opposing portions of the distal tip section.
The axes of the sensing coils are parallel to the catheter axis (and thus to one another, when the joint is undeflected). Consequently, the sensing coils are configured to output strong signals in response to the field generated by the miniature field generator. The signals vary strongly with the distances of the coils. Angular deflection of the distal portion carrying the miniature field generator gives rise to a differential change in the signals output by sensing coils, depending on the direction and magnitude of deflection, since one or two of these coils move relatively closer to the field generator. Compressive displacement of the distal portion gives rise to an increase in the signals from all of three sensing coils. Prior calibration of the relation between pressure on distal portion and movement of joint may be used by processor in translating the coil signals into terms of pressure. By virtue of the combined sensing of displacement and deflection, the sensors read the pressure correctly regardless of whether the electrode engages the endocardium head-on or at an angle.
With position sensing and pressure sensing, a conventional catheter may carry six leads, one for each of the three position sensing coils and the three pressure sensing coil, with each lead being a twisted pair of wires. Leads are time-consuming and expensive to manufacture and install. Moreover, the leads occupy space in the space-constrained catheter tip and are susceptible to breakage. A reduction in the number of leads used in the catheter and/or their lengths would provide a number of benefits, including reduced catheter production time, increased total catheter yield, and reduced production costs.
Some catheterization procedures require the use of a second catheter in close proximity to a first catheter. Shaft Proximity Interference (“SPI”) occurs when metal components of the second catheter disturb sensing coils in the first catheter. For example, where a pressure sensing coil reacts to changes in the magnetic field due to errant magnetic interference by an adjacent catheter instead of physical distortion of a distal tip due to tissue contact, signals from the coil can mislead an operator relying a catheterization system processing those signals.
Accordingly, it is desirable to provide a catheter with combined or simplified position and pressure sensing capabilities for reducing the number of sensor coil leads and/or their lengths. It is also desirable to provide a catheter capable of recognizing distortions in magnetic fields caused by factors other than physical distortion of the distal tip due to tissue contact.